1. Field of the Invention
The present invention relates to a method and apparatus for forming a semiconductor thin film on a base layer made of an insulating material.
2. Prior Art
It is known well to form a semiconductor thin film on a base layer by mean of a laser crystallization method. More particularly, in this method, the base layer made of an insulating material is first prepared for instance in the form of an amorphous substrate, or especially in the form of a low cost glass substrate. Then, there is formed on the above substrate a semiconductor thin film having excellent crystallinity like a silicon (Si) thin film. Furthermore, this thin film is processed by means of an ultraviolet (UV) pulse laser, thereby the semiconductor thin film being poly-crystallized, eventually. The method of this kind is already fitted for practical use.
However, a silicon thin film produced by the currently available laser crystallization technique is a poly-crystal thin film, of which the average grain size is several hundreds nanometer (nm) and the mobility is 200 m2/V∘sec at the most due to the influence of a grain boundary.
In a thin film transistor using these thin films, the channel length L of the transistor has to be ten times or more as long as the grain size, that is, about several μm if taking account of dispersion in the electric performance of the thin film transistor. As a result, a circuit which can be designed by using this transistor is only a driving circuit of which the cutoff frequency is 5 MHz or so at the most.
If intending to design such a high performance driving circuit that is operable at a frequency of 100 MHz, it will be roughly estimated that a thin film transistor has to have a channel length of 1 μm and the thin film constituting it has to have a mobility of 300 m2/V∘sec or so. Moreover, the transistor has to have the least or no dispersion in its electric performance. In other words, the semiconductor thin film (Si thin film) formed on the amorphous substrate is required to have the grain size of 1 μm or more and also to have no grain boundary in the channel to be formed in the above thin film.
As a laser crystallization method satisfying the above requirements, there have been proposed a Sequential Lateral Solidification (SLS) Method (the first prior art), and a Phase Sifter Crystallization Method (the second prior art).
<<First Prior Art>>
The SLS method is made up of combination of the Super Lateral Growth (SLG) phenomena and the Step-and-Repeat method taken at a stage as needed.
Referring to FIG. 7, a reference numeral 71 indicates an excimer laser, 72 an emitted laser beam, 73 a laser homogenizer, 74 a line beam (homogenized laser light), 75 an amorphous substrate, 76 a non-single crystal semiconductor layer, and 77 a poly-crystallized semiconductor layer.
Heretofore, the semiconductor thin film for use in the thin film transistor as used in a liquid crystal display has been made of an amorphous silicon thin film. In general, the mobility in the amorphous silicon thin film is about 1 cm2/V∘sec. This value is enough for a switching transistor for use in a liquid crystal display panel of the active matrix type. However, as a result of recent research and development for improving the performance of the thin film transistor formed on the glass substrate, it has been made possible to form a silicon thin film having a mobility of 100 cm2/V∘sec even on the amorphous silicon by the thin film crystallization technique using the excimer laser as shown in FIG. 7. The thin film obtained by this crystallization technique is a poly-crystal thin film having a grain size “a” of about 300 to 500 nm. In the crystallization method using the excimer laser, the ultraviolet radiation is given to only the silicon thin film for such a very short period of time as 20 nsec or so, thereby only the silicon thin film being crystallized through the process of being melted and solidified. Consequently, as the radiation period of time is so short, there is less or no chance that the thermal damage is caused to the substrate.
In the apparatus shown in FIG. 7, a light source is constituted by a high-power pulse laser such as a xenon chloride (XeCl) laser (wavelength: 308 nm). The output form of the laser light used in the mass production process is a rectangle with a size of 2 cm×1 cm. Usually, the laser beam of this form is further processed to form a line beam of 20 cm (length b)×300 to 500 μm (width a) and at the same time, the intensity of this beam is homogenized by the homogenizer 73. A glass plate made of a parent material glass for use in the liquid crystal display is fed with a feed-pitch of 10 to 20 μm, thereby the amorphous silicon film formed on the parent glass plate being entirely crystallized.
Referring to FIG. 8, a reference numeral 72 indicates a laser beam emitted from an excimer laser, 81 a fly-eye lens of a homogenizer (73 in FIG. 7), 74 a line beam, and 82 a light projection optical system (not shown in FIG. 7)
The laser beam 72 emitted from the excimer laser have a rectangular form of 2 cm×1 cm as described in the above. The excimer laser is a considerably uniform light source comparing with an ordinary solid-state laser, but as shown in FIG. 8, it is observed that the light intensity slowly goes down in the vicinity of the edge. As shown in FIG. 8, the laser homogenizer 73 (FIG. 7) used in the above first prior art is able to divide the laser beam and to change the beam form by using the fly-eye lens 81, and further to improve the homogeneity of the beam intensity. Accordingly, if the semiconductor thin film formed on a large area substrate is scanned at a pitch of 10 to 20 μm by using the line beam 74 obtained by the way as shown in FIG. 8, a semiconductor thin film cab can be crystallized on the large area substrate.
However, the technique making use of the SLG region for obtaining a high performance Si thin film based on the basis of the first prior art shown in FIGS. 7 and 8, or on the other prior arts, has some drawbacks as described in the following, which are:
1) It is theoretically impossible to execute the step-and-repeat method when the feed-pitch exceeds the length (at most 1 μm) of the SLG. It is hardly possible to expect any improvement in productivity, accordingly.
2) There are certain restrictions on the mobility of a poly-crystal thin film as formed by the above technique. In case of a poly-crystal thin film which has grown up by allowing the grain size to become larger without controlling the position of the grain boundary, the dispersion in the grain size becomes larger. Thus, the above technique is far from practical use.
3) Residual grain boundaries exist at an interval of about several hundreds nm in the scanning direction while in the direction perpendicular to the scanning direction, crystal (lattice) defects exist at an interval of a feed-pitch. Accordingly, for the time being, it would be not suitable to apply the above technique to the thin film transistor of which the channel has a length of 1 μm.
<<Second Prior Art>>
In the above-mentioned phase shifter crystallization method, the light irradiation intensity on the substrate is varied by means of a phase shifter capable of changing the phase of at least a part of the light with reference to a predetermined design of light irradiation intensity, thereby controlling the lateral crystal growth and obtaining a crystal having a large crystal grain size. Especially, with regard to this method, there is a disclosure by Matsumura el al, which discloses its basic concept and theoretical verification in the article entitled “Preparation of Ultra-Large Grain Silicon Thin Film by Excimer-Laser” (Surface Science Vol. 21, No. 5, pp. 278-287, 2000).
Referring to FIG. 9(a), a reference numeral 91 indicates an excimer laser, 92 an emitted laser beam, 93 a beam intensity conversion optical system for converting laser beam intensity (dimension), 94 and 95 a phase shifter, 96 an amorphous substrate, and 97 a non-single crystal semiconductor layer. In FIG. 9(b), 98 indicates a start point of crystal growth, and 99 a single crystal grain.
As described in connection with the first prior art, with recent further technical progress related to the thin film formation on the glass substrate, it becomes practically possible to produce a thin film of which the mobility is about 100 m2/V∘sec. Accordingly, it becomes possible to integrate the thin film transistor for a driving circuit and the thin film transistor for the pixel use on an identical glass substrate. In order to systemize the liquid crystal display and so forth, however, it is still requested to find other materials more suitable for the thin film transistor showing high performance and less dispersed characteristics. The second prior art shown in FIG. 9 shows a technique in compliance with the above request. That is, it is a technique for controlling the crystal grain size (to the extent of 5 μm) as well as the position of the crystal grain boundary. In this example, the beam 92 emitted from the excimer laser 91 is basically used as the light source as it is. If, however, the light intensity is not obtained sufficiently, the beam form is converted by the beam intensity conversion optical system 93 (described in detail referring to FIG. 10, later) and this converted beam is used. The most important point of this technique is the point that the light intensity is two-dimensionally modulated by means of two phase shifters 94 and 95 arranged to take a position perpendicular to each other. That is, the phase shifter 94 carries out the comparatively soft modulation (10 μm pitch) in the direction of an arrow A (the scanning direction of the glass substrate) in FIG. 9(a) while the phase shifter 95 performs the modulation (d=20 μm: a now actually proved value) in the direction of an arrow B (perpendicular to the scanning direction of the glass substrate). With combination of these modulations, the start point 98 of the crystal growth is produced while the crystal lateral growth is induced in the arrow “A” direction as shown in FIG. 9(b) (described in detail referring to FIG. 11, later).
Referring to FIG. 10, a reference numeral 92 indicates a beam emitted from the excimer laser, 93 a beam intensity conversion optical system and 100 a mask (diaphragm).
As already described, the laser beam 92 emitted from the excimer laser has a rectangular form of 2 cm×1 cm and also has a considerably good uniformity comparing with those which are emitted from a solid-state laser. As shown in FIG. 10, however, it is observed that the light intensity slowly goes down in the vicinity of the edge. In the second prior art, since two phase sifters 94 and 95 are used and the spatial coherence of the beam is required, it is needed to use such an optical system as uses a single lens or combination thereof as shown in FIG. 9. In order to convert the light irradiation intensity, the beam diameter is converted by using the beam intensity conversion optical system 93 as shown in FIG. 10. With this, the spatial coherence of the beam may be maintained, but homogeneity of the beam can not be improved. This is one problem of the technique as used in the second prior art. To solve the problem, there is provided a mask (diaphragm) 100 as shown in FIG. 10. The mask 100 might reduce the usage efficiency of the light, but it improves the homogeneity of the light.
Referring to FIG. 11(a), reference numerals 94 and 95 indicate the phase shifters, respectively, 96 an amorphous substrate, 97 a non-single crystal semiconductor layer, and 90 an excimer laser light. Referring to FIG. 11(b), a reference numeral 98 indicates the start point of the crystal growth and 99 a single crystal grain.
It has been described that the most important point of the second prior art is the point that the light intensity is two-dimensionally modulated. As shown in 11(a), the phase shifter 94 (Y-shifter) can modulates the excimer laser light 90 to cause the light intensity modulation as shown by ↑ in FIG. 11(b) while the phase shifter 95 (X-shifter) can modulate the excimer laser light 90 to cause the light intensity modulation as shown by → in FIG. 11(b). If these two separate phase shifters are arranged to direct to the directions perpendicular to each other, it becomes possible to grow the single crystal grain 99 of the position control type as shown in FIG. 11(b).
As shown in FIGS. 9 to 11, however, the second prior art has the following defects, which are:
1) As the light irradiation intensity on the substrate is varied by means of the phase shifters 94 and 95 capable of changing the phase of at least a part of the light with reference to a predetermined design of light irradiation intensity, there might be obtained the lateral crystal growth to the extent of about 5 to 10 μm. In this case, however, as it never fails to happen that some regions are left without being crystallized in the form of a single crystal, the complete high density crystal would not be obtained.
2) As the phase shifters 94 and 95 are used, it is required for the irradiation light to be of coherence and also, it is needed for the laser to emit parallel beams. The excimer laser capable of supplying a high-power now on market has an angle of divergence and is in the trade-off relation with regard to the relation between the positional accuracy and the lateral growth length. In addition, as the excimer laser handles a parallel beam system, the homogeneity of the beam amplitude depends on the amplitude intensity distribution of the beam immediately after emitted from the laser cavity resonator.
Because of this, the problems to be solved still remain with regard to the positional accuracy, the high density crystallization, and so forth in the region to be crystallized, and the trade-off relation comes to occur between the homogeneity in the laser irradiation region and the irradiation area. Thus, the practical use of the second prior art might invite a new problem in view of the productivity.
Accordingly, an object of the invention is to provide a method for forming a semiconductor thin film having excellent crystallinity on a base layer made of an insulating material and also to provide apparatus capable of performing the method.